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Register a new userPre-saturation phase in the frustrated ferro-antiferromagnet Pb$_2$VO(PO$_4$)$_2$
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F. Landolt
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Magnetization, magnetic torque, neutron diffraction and NMR experiments are used to map out the $H$$-$$T$ phase diagram of the prototypical quasi-two-dimensional ferro-antiferromagnet Pb$_2$VO(PO$_4$)$_2$ in magnetic fields up to 27 T. When the field is applied perpendicular to the axis of magnetic anisotropy, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. The measured NMR spectra suggest a complex and possibly incommensurate magnetic order in that regime.
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Single crystal neutron diffraction, inelastic neutron scattering and electron spin resonance experiments are used to study the magnetic structure and spin waves in Pb$_2$VO(PO$_4$)$_2$, a prototypical layered $S=1/2$ ferromagnet with frustrating next nearest neighbor antiferromagnetic interactions. The observed excitation spectrum is found to be inconsistent with a simple square lattice model previously proposed for this material. At least four distinct exchange coupling constants are required to reproduce the measured spin wave dispersion. The degree of magnetic frustration is correspondingly revised and found to be substantially smaller than in all previous estimates.
Magnetic properties and magnetic structure of the Ba$_{2}$Mn(PO$_{4}$)$_{2}$ antiferromagnet featuring frustrated zigzag chains of $S=frac{5}{2}$ Mn$^{2+}$ ions are reported based on neutron diffraction, density-functional band-structure calculations, as well as temperature- and field-dependent measurements of the magnetization and specific heat. A magnetic transition at $T_Nsimeq 5$,K marks the onset of the antiferromagnetic order with the propagation vector ${mathbf k} = (frac12, 0, frac12)$ and ordered moment of $4.33pm0.08~mu_B$/Mn$^{2+}$ at 1.5,K, pointing along the $c$ direction. Direction of the magnetic moment is chosen by the single-ion anisotropy, which is relatively weak compared to the isostructural Ni$^{2+}$ compound. Geometrical frustration has strong impact on thermodynamic properties of Ba$_2$Mn(PO$_4)_2$, but manifestations of the frustration are different from those in Ba$_2$Ni(PO$_4)_2$, where frustration by isotropic exchange couplings is minor, yet strong and competing single-ion anisotropies are present. A spin-flop transition is observed around 2.5,T. The evaluation of the magnetic structure from the ground state via the spin-flop state to the field-polarized ferromagnetic state has been revealed by a comprehensive neutron diffraction study as a function of magnetic field below $T_N$. Finally, a magnetic phase diagram in the $H-T$ plane is obtained.
We have found an unusual competition of two frustration mechanisms in the 2D quantum antiferromagnet Cs$_2$CoBr$_4$. The key actors are the alternation of single-ion planar anisotropy direction of the individual magnetic Co$^{2+}$ ions, and their arrangement in a distorted triangular lattice structure. In particular, the uniquely oriented Ising-type anisotropy emerges from the competition of easy-plane ones, and for a magnetic field applied along this axis one finds a cascade of five ordered phases at low temperatures. Two of these phases feature magnetization plateaux. The low-field one is supposed to be a consequence of a collinear ground state stabilized by the anisotropy, while the other plateau bears characteristics of an up-up-down state exclusive for lattices with triangular exchange patterns. Such coexistence of the magnetization plateaux is a fingerprint of competition between the anisotropy and the geometric frustration in Cs$_2$CoBr$_4$.
There is strong interest in discovering or designing wide gap Chern insulators. Here we follow a Chern insulator to trivial Mott insulator transition versus interaction strength U in a honeycomb lattice Fe-based transition metal oxide, discovering that a spin-orbit coupling energy scale $xi$=40 meV can produce and maintain a topologically entangled Chern insulating state against large band structure changes arising from an interaction strength U up to 60 times as large. Within the Chern phase the minimum gap switches from the zone corner K to the zone center $Gamma$ while maintaining the topological structure. At a critical strength $U_c$, the continuous evolution of the electronic structure encounters a gap closing then reopening, upon which the system reverts to a trivial Mott insulating phase. This Chern insulator phase of honeycomb lattice Fe$^{2+}$ BaFe$_2$(PO$_4$)$_2$ corresponds to a large Chern number C=-3 that will provide enhanced anomalous Hall conductivity due to the associated three edge states threading through the bulk gap of 80 meV.
We present observations of highly frustrated quasi two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the S$_{eff}$ = 1/2 compound $gamma$-BaCo$_2$(PO$_4$)$_2$ ($gamma$-BCPO). Specific heat shows a broad peak comprised of two weak kink features at $T_{N1} sim$ 6 K and $T_{N2} sim$ 3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below $T_{N1}$ and $T_{N2}$, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length $xi_c = 60pm2$ AA ($T_{N1}$) and in quasi-2D helical domains with $xi_h = 350 pm 11$ AA ($T_{N2}$). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on $gamma$-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ $J_1$-$J_2$-$J_3$ model with ferromagnetic nearest-neighbor exchange $J_1$ are favored, both near regions of high classical degeneracy. High energy coherent excitations ($sim 10$ meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above $T_N$. These data show that $gamma$-BCPO is a rare highly frustrated, quasi-2D S$_{eff}$ = 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.
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